Thermal flow sensor

ABSTRACT

A thermal flow sensor can adjust the temperature of a heating element in a highly precise and reliable manner with the use of simple circuits, devices and process steps. The sensor includes an amplifier section ( 7 ) that amplifies a voltage across opposite ends of at least one of resistors ( 3, 4, 5 ) that constitute a bridge circuit, a current control section ( 9 ) that is controlled based on an output voltage of the amplifier section ( 7 ), and an output terminal ( 14 ) that is connected to one end of a heating element ( 1 ) that is controlled to be energized by the current control section ( 9 ). The amplifier section ( 7 ) includes an amplification factor control section for controlling an amplification factor by an electric signal from a computer, and uses an output voltage which has been amplified and impressed to an input voltage to an operational amplifier ( 8 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal flow sensor for detecting theflow rate of fluid by electrically detecting the amount of heattransmitted to the fluid from a heating element arranged therein.Particularly, the invention relates to a novel improvement in theconfiguration of a circuit for adjusting the temperature of the heatingelement.

2. Description of the Related Art

In general, in thermal flow sensors, a heating element and a fluidtemperature detection element are arranged in fluid, and a bridgecircuit is formed by the heating element, the fluid temperaturedetection element, and a plurality of resistors, where about in aheating current supplied to the heating element is controlled so as tomake the bridge circuit always keep equilibrium.

As a result, the temperature of the heating element is always kept at acontrol temperature that is higher by a predetermined temperature thanthe temperature of the fluid detected by the fluid temperature detectionelement.

However, there exist variations in the heating element, the fluidtemperature detection element, the registers, the sensor structure,etc., respectively, so it is necessary to adjust the predeterminedtemperature for each of individual thermal flow sensors.

In the known thermal flow sensors, to adjust the heating element to thecontrol temperature, the resistance value of at least one of theresistors forming the bridge circuit is adjusted.

As a adjusting method for such a resistance value, there has been widelyused a method in which a resistance value necessary to obtain thepredetermined temperature is calculated, and a fixed resistor having aresistance value close to the resistance value thus calculated issoldered to the bridge circuit, or a method of using a variable resistorwhose resistance value can be changed by a driver or the like, or alaser trimming method in which a part of a membrane resistor is burn offby a laser so as to change its resistance value (see, for example, afirst patent document: Japanese patent application laid-open No.2000-314645).

In the known thermal flow sensors, in case where the fixed resistor issoldered for example, it is necessary to perform the calculation of anoptimal resistance value, the selection of an appropriate resistor, andsoldering work, so there is a problem that the number of steps requiredbecomes large, and much time is required for adjustment.

In addition, in case where the variable resistor is used, there arisesanother problem that the resistance value of the variable resistor canbe changed due to vibration, etc., thus resulting in a lack of accuracy.

Moreover, in case where laser trimming is applied, the apparatus becomeslarge in scale, and besides, the resistance value can be adjusted onlyin a direction from a small resistance value to a large resistancevalue, so an initial temperature might become too high in considerationof the case where the temperature of the heating element is to bechanged in a direction from a high temperature side to a low temperatureside, and there is a further problem that when an optimal value has oncebeen exceeded, no readjustment can be made.

SUMMARY OF THE INVENTION

Accordingly, the present invention is intended to obviate the problemsas referred to above, and has for its object to obtain a thermal flowsensor which is provided with an adjustment circuit capable of adjustingthe temperature of a heating element in the thermal flow sensor in ahighly precise and reliable manner with the use of simple circuits,devices and steps.

Bearing the above object in mind, according to the present invention,there is provided a thermal flow sensor in which a bridge circuit iscomposed of at least one heating element, a fluid temperature detectionelement, and a plurality of resistors; the heating element and the fluidtemperature detection element are arranged in fluid, and the fluidtemperature detection element is arranged at a location free from theinfluence of heat from the heating element; and a flow rate of the fluidis detected by using the fact that an amount of heat transmitted fromthe heating element to the fluid in a state where the heating element isalways held at a control temperature higher by a predetermined valuethan the temperature of the fluid detected by the fluid temperaturedetection element depends on the flow rate of the fluid. The thermalflow sensor includes: an amplifier section that amplifies a voltageacross opposite ends of at least one of the plurality of resistors; acurrent control section that is controlled based on an output voltage ofthe amplifier section; and an output terminal that is connected to oneend of the heating element, which is controlled to be energized throughthe current control section, for outputting a detection resultcorresponding to the flow rate of the fluid. The amplifier sectionincludes an amplifier part that amplifies an input signal to theamplifier section, and an amplification factor control part thatcontrols an amplification factor of the amplifier part. Theamplification factor control part changes the amplification factor ofthe amplifier part by means of an electric signal so that thetemperature of the heating element is adjusted to the controltemperature.

According to the present invention, it is possible to adjust thetemperature of the heating element in the thermal flow sensor in ahighly precise and reliable manner with the use of simple circuits,devices and steps.

The above and other objects, features and advantages of the presentinvention will become more readily apparent to those skilled in the artfrom the following detailed description of preferred embodiments of thepresent invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit configuration diagram showing a thermal flow sensoraccording to a first embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing an a rich amplifiersection in FIG. 1.

FIG. 3 is a circuit diagram showing one example of an equivalent circuitof the amplifier section shown in FIG. 2.

FIG. 4 is a circuit configuration diagram showing an amplifier sectionaccording to a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing one example of an equivalent circuitof the amplifier section shown in FIG. 4.

FIG. 6 is a circuit configuration diagram showing an amplifier sectionaccording to a third embodiment of the present invention.

FIG. 7 is a circuit configuration diagram showing an amplifier sectionaccording to a fourth embodiment of the present invention.

FIG. 8 is a circuit diagram showing one example of an equivalent circuitof the amplifier section shown in FIG. 7.

FIG. 9 is a circuit configuration diagram showing a thermal flow sensoraccording to the fourth embodiment of the present invention.

FIG. 10 is an explanatory view showing a mounting state of a thermalflow sensor according to a fifth embodiment of the present invention.

FIG. 11 is an explanatory view showing a mounting state of a thermalflow sensor according to a sixth embodiment of the present invention.

FIG. 12 is a circuit configuration diagram showing an amplifier sectionaccording to a seventh embodiment of the present invention.

FIG. 13 is a circuit configuration diagram showing a thermal flow sensoraccording to an eighth embodiment of the present invention.

FIG. 14 is a circuit configuration diagram showing a thermal flow sensoraccording to a ninth embodiment of the present invention.

FIG. 15 is a circuit configuration diagram showing a thermal flow sensoraccording to a tenth embodiment of the present invention.

FIG. 16 is a circuit configuration diagram showing a thermal flow sensoraccording to an eleventh embodiment of the present invention.

FIG. 17 is a circuit configuration diagram showing a thermal flow sensoraccording to a twelfth embodiment of the present invention.

FIG. 18 is a circuit configuration diagram showing a thermal flow sensoraccording to a thirteenth embodiment of the present invention.

FIG. 19 is a circuit configuration diagram showing a thermal flow sensoraccording to the fourteenth embodiment of the present invention.

FIG. 20 is a circuit configuration diagram showing a thermal flow sensoraccording to a fifteenth embodiment of the present invention.

FIG. 21 is a circuit configuration diagram showing a thermal flow sensoraccording to a sixteenth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail while referring to the accompanying drawings.

Embodiment 1

FIG. 1 is a circuit configuration diagram that shows a thermal flowsensor according to a first embodiment of the present invention, whereinthe configuration of a temperature control circuit of a heating elementis specifically illustrated.

In FIG. 1, a bridge circuit includes a heating element 1 that isarranged in fluid, a fluid temperature detection element 2 that isarranged in the fluid at a location free from the influence of heat fromthe heating element 1, and a plurality of resistors 3, 4, 5.

A first series circuit comprising the heating element 1 and the resistor5 and a second series circuit comprising the fluid temperature detectionelement 2 and the resistors 3, 4 are connected in parallel with eachother, and inserted between a transistor 9 connected to a power supply10 and ground.

The heating element 1 and the fluid temperature detection element 2 areformed of a temperature sensitive resistance material such as forexample platinum, nickel, etc., whose resistance value changes inaccordance with the temperature thereof.

A buffer circuit 6, an amplifier section 7 and an operational amplifier8 are connected to the bridge circuit, and the operational amplifier 8has an output terminal connected to a base terminal of the transistor 9.

A voltage across the opposite ends of the resistor 4 is impressed toinput terminals 17, 18 of the amplifier section 7 through the buffercircuit 6. The input terminal 18 of the amplifier section 7 is connectedto the ground, and the output terminal 19 of the amplifier section 7 isconnected to an inverting input terminal 16 of the operational amplifier8.

Here, note that though not shown in FIG. 1, a computer 22 (to bedescribed later) such as a personal computer, which serves as externalequipment for sensor adjustment, is connected to the amplifier section 7at the time of adjustment of the thermal flow sensor.

A voltage at a junction 12 between the heating element 1 and theresistor 5 is impressed to a non-inverting input terminal 15 of theoperational amplifier 8, and the junction 12 of the heating element 1and the resistor 5 is connected to an output terminal 14 of the thermalflow sensor, so that the output terminal 14 outputs a detection resultcorresponding to the flow rate of the fluid.

In this case, the amplifier section 7 is designed to divide a voltageinput thereto into a ratio of “x:1−x” and output the thus dividedvoltage. An output voltage of the operational amplifier 8 based on theoutput voltage of the amplifier section 7 is impressed to the baseterminal of the transistor 9, whereby the transistor 9 controls acurrent supplied to the heating element 1 based on the voltage input tothe base terminal thereof. As a result, the heating element 1 iscontrolled to be energized and deenergized through the transistor 9 thatis current controlled by the operational amplifier 8 (i.e., operates inan active region).

FIG. 2 is a circuit configuration diagram that specifically illustratesa state in which the amplifier section 7 in FIG. 1 is connected to thecomputer 22 (external equipment for adjustment).

In FIG. 2, the amplifier section 7 is composed of an amplifier part 20and an amplification factor control part 23, which are arranged betweenthe input terminals 17, 18 and the output terminal 19 of the amplifiersection 7. The amplifier part 20 and the amplification factor controlpart 23 together constitute a so-called DA converter. The amplificationfactor control part 23 performs an adjustment operation by means of anelectric signal Q from the computer 22 (external equipment foradjustment). Connected to the amplification factor control part 23 is amemory 21 that serves to store the electric signal Q after completion ofthe adjustment as data, and the electric signal Q after adjustment issupplied to the amplification factor control part 23.

The amplifier part 20 is composed of a resistor array comprising aplurality of resistors 2R0 through 2R4 connected between the inputterminals 17, 18 and the output terminal 19 of the amplifier section 7,and resistors R10 through R30 inserted between adjacent junctions of therespective resistors 2R1 through 2R4. The resistor 2R0 has one endthereof connected to the input terminal 18, and the resistor 2R4 has theother end thereof connected to the output terminal 19.

The amplification factor control part 23 is composed of a plurality ofswitches S1 through S4 that can be electrically controlled. Theindividual switches S1 through S4 in the amplification factor controlpart 23 serve to connect, in response to the electric signal Q from thecomputer 22, one ends of the respective resistors 2R1 through 2R4 in theamplifier part 20 to either one of the input terminals 17, 18.

In FIG. 2, the amplifier section 7 divides the voltage input thereto(i.e., amplifies it at a ratio of 1 or less), and outputs the thusdivided voltage from the output terminal 19 as an output voltage. Also,in the amplifier part 20, the resistance values of the respectiveresistors 2R0 through 2R4 are represented by “2R”, and the resistancevalues of the resistors R10 through R30 are represented by “R”.

Although in FIG. 2, a DA converter of 4 bits is shown as a configurationexample of the amplifier section 7, it is needless to say that thenumber of bits of the DA converter can be arbitrarily changed asrequired. Also, the amplifier section 7 is shown as being composed ofthe DA converter of the R−2R type, but it may be composed of a DAconverter of another type. Such a situation is similar in respectiveembodiments to be described later.

FIG. 3 is a circuit configuration diagram that illustrates one exampleof the amplifier part 20, wherein there is illustrated an equivalentcircuit in case where f or example, the resistors 2R1 through 2R3 areconnected to the input terminal 18 by means of the switches S1 throughS3, respectively, in FIG. 2, with only the resistor 2R4 being connectedto the input terminal 17 by means of the switch S4. In FIG. 3,individual resistance values or combined resistance values arerepresented by “2R”.

In this case, an output voltage V19 from the output terminal 19 isrepresented by the following expression (1) by using individual inputvoltages V17, V18 of the input terminals 17, 18. $\begin{matrix}{{V\quad 19} = {{{\frac{2R}{4R}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}} = {\frac{1}{2}\left( {{V\quad 17} + {V\quad 18}} \right)}}} & (1)\end{matrix}$

As can be seen from expression (1), the voltage division ratio x of theamplifier section 7 is decided by the ratio of resistance values(=2R/4R=½), so the absolute accuracy of the resistance values is notrequired, and voltage division with high precision can be made as longas the relative accuracy thereof is kept.

Here, note that the general formula of the output voltage V19 becomes,as shown by the following expression (2). $\begin{matrix}{{V\quad 19} = {{\frac{{V\quad 17} - {V\quad 18}}{2^{4}} \times N} + {V\quad 18}}} & (2)\end{matrix}$where N is a value which represents the connection states of theswitches S1 through S4 by the digital value of a decimal number.

Specifically, when a switch is connected to the input terminal 17 sidle,this state is represented by “1”, whereas when a switch is connected tothe input terminal 18 side, this state is represented by “0”, and theswitches S1, S4 are represented by “LSB” and “MSB”, respectively.

When the switch S4 alone is connected to the input terminal 17 side asin the above example, this state is represented by N=1000 (binarynumber)=8 (decimal number). Accordingly, by assigning N=8 to expression(2), the output voltage V19 is represented by V19=(V17+V18)/2.

In addition, when the amplifier section 7 divides the input voltage intoa ratio of “x:1−x”, and outputs the thus divided voltage, as shown inFIG. 1, an input voltage V16 to the inverting input terminal 16 of theoperational amplifier 8 is represented by using a voltage V13 at ajunction 13 between the fluid temperature detection element 2 and theresistor 4, as shown by the following expression (3).V16=(1−x)V13  (3)

Moreover, the voltage V13 is represented by using the voltage V11 at ajunction 11 between the heating element 1 and the resistor 3, as shownby the following expression (4). $\begin{matrix}{{V\quad 13} = {\frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (4)\end{matrix}$where R2, R3 and R4 are the individual resistance values of the fluidtemperature detection element 2 and the resistors 3, 4, respectively.

Further, an input voltage V15 to the non-inverting input terminal 15 ofthe operational amplifier 8 is represented by the following expression(5). $\begin{matrix}{{V\quad 15} = {\frac{R\quad 5}{{R\quad 1} + {R\quad 5}}V\quad 11}} & (5)\end{matrix}$where R1 and R5 are the individual resistance values of the heatingelement 1 and the resistor 5, respectively.

Here, if the input terminals 15, 16 of the operational amplifier 8 arevirtually short-circuited with each other, the input voltages V15, V16to the input terminals 15, 16 become equal to each other (V15=V16), andhence the above expression (5) is represented by the followingexpression (6). $\begin{matrix}{{\frac{R\quad 5}{{R\quad 1} + {R\quad 5}}V\quad 11} = {\left( {1 - x} \right)\frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (6)\end{matrix}$

Accordingly, the resistance value R1 of the heating element 1 isrepresented by the following expression (7). $\begin{matrix}{{R\quad 1} = \frac{\left( {{R\quad 2} + {R\quad 3} + {{xR}\quad 4}} \right)R\quad 5}{\left( {1 - x} \right)R\quad 4}} & (7)\end{matrix}$

As can be seen from expression (7), by changing the voltage divisionratio x of the amplifier section 7, the resistance value, i.e., thetemperature, of the heating element 1 can be adjusted. The voltagedivision ratio x is changed by an electric signal Q input to theamplification factor control part 23 in FIG. 2.

At the time of adjustment of the amplification factor control part 23,the electric signal Q is sent from the computer 22 so as to adjust theindividual switches S1 through S4, and when an optimal adjustment valueis finally decided or fixed, the electric signal Q after the finaldecision of adjustment is stored in the memory 21 as an adjustmentsignal. After adjustment, by reading out an electric signal foradjustment from the memory 21 and sending it to the amplification factorcontrol part 23, the amplification factor optimally adjusted can bereproduced. This is similar in respective embodiments to be describedlater. Here, note that the output voltage V19 of the amplifier section 7is not only used as an input voltage to the operational amplifier 8 butalso can be used as a voltage impressed to the other side of the bridgecircuit.

As described above, according to the first embodiment of the presentinvention, as the temperature control circuit of the thermal flowsensor, there is provided the amplifier section 7 that serves to amplifya voltage across the opposite ends of at least one resistor among theplurality of resistors that constitute the bridge circuit, and theamplifier section 7 includes the amplifier part 20 and the amplificationfactor control part 23 that serves to control the amplification factorof the amplifier part 20 by an electric signal Q from the computer 22,with the amplified output voltage V19 being used as an input voltage ofthe operational amplifier 8. With such an arrangement, the temperatureof the heating element 1 can be adjusted only by sending the electricsignal Q for adjustment from the computer 22, so the adjustment can bemade in a short time by a simple arrangement or configuration of theapparatus.

That is, by changing the amplification factor of the amplifier section 7for each of the thermal flow sensors with individual variations by meansof the electric signal Q from the computer 22, it is possible to adjustthe temperature of the heating element 1 to a predetermined controltemperature, so apparatuses for mechanical processing (soldering, lasertrimming, etc.) become unnecessary. Besides, adjustment can be madewhile monitoring the temperature of the heating element 1 (or an indexindicating the temperature), so an adjustment process or step becomessimple.

In addition, since the voltage division ratio x of the amplifier section7 is decided by the ratio of the resistance values, as stated above,adjustment accuracy is not influenced by the absolute accuracy of theresistance values, so only the relative accuracy thereof need to bemaintained, and adjustment with a high degree of precision can beachieved even if highly accurate and expensive elements are not used.

Further, the amplification factor control part 23 is composed of theswitches S1 through S4 that can electrically be opened and closed, anddata of the electric signal Q to achieve the amplification factor aftercompletion of the adjustment is stored in the memory 21, after which thedata thus stored is used by being read out from the memory 21, wherebythe temperature of the heating element 1 can be adjusted accurately bythe use of a simple circuit configuration, and a thermal flow sensorwith a high degree of precision and reliability can be achieved.

Embodiment 2

Although in the above-mentioned first embodiment, the amplifier part 20comprising the resistor array is used, an amplifier part 20A comprisinga capacitor array may be used, as shown in FIG. 4.

FIG. 4 is a circuit configuration diagram that shows an amplifiersection 7A according to a second embodiment of the present invention,wherein the like parts or components as those described above (see FIG.2) are identified by the same symbols or by the same symbols with “A”affixed to their ends, while omitting a detailed explanation thereof.Also, the circuit configuration of the sensor as a whole is as shown inFIG. 1.

In FIG. 4, the amplifier part 20A is composed of a capacitor arraycomprising a plurality of capacitors C0, C1, 2C, 4C, 8C connectedbetween input terminals 17, 18 and an output terminal 19 of theamplifier section 7A. The capacitor C0 has one end thereof connected tothe input terminal 18 of the amplifier section 7A, and the capacitorsC1, 2C, 4C, 8C have one ends thereof connected to the switches S1through S4, respectively, in the amplification factor control part 23.In addition, the capacitors C0, C1, 2C, 4C, 8C have their commonterminals connected to a non-inverting input terminal (+) of anoperational amplifier 24, and hence connected to the output terminal 19of the amplifier section 7A through the operational amplifier 24. Theoperational amplifier 24 has an output terminal thereof connected to itsown inverting input terminal (−).

Here, note that the capacitance value of each of the capacitors C0, C1is represented by “C”; the capacitance value of the capacitor 2C isrepresented by “2C”; the capacitance value of the capacitor 4C isrepresented by “4C”; and the capacitance value of the capacitor 8C isrepresented by “8C”.

The amplifier part 20A and the amplification factor control part 23shown in FIG. 4 have a configuration similar to that of a so-calledcharge distribution type DA converter. FIG. 5 is a circuit configurationdiagram that illustrates one example of the amplifier part 20A, whereinthere is illustrated an equivalent circuit in case where for example,the capacitors C1, 2C, 4C are connected to the input terminal 18 bymeans of the switches S1 through S3, respectively, in FIG. 4, with onlythe capacitor 8C being intermittently connected to the input terminal 17by means of the switch S4. In FIG. 8, the capacitance value or thecombined capacitance value is represented by “8C”.

At this time, the output voltage V19 from the output terminal 19 isrepresented by the following expression (8). $\begin{matrix}{{V\quad 19} = {{{\frac{8\quad C}{16\quad C}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}} = {{\frac{1}{2}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}}}} & (8)\end{matrix}$

As can be seen from expression (8), the voltage division ratio x of theamplifier section 7A is decided by the ratio of the capacitance values(=8C/16C=½), and the absolute accuracy of the capacitance values is notrequired, so voltage division with high precision can be made as long asthe relative accuracy thereof is kept.

Here, note that the general formula of the output voltage V19 becomes,as shown by the following expression (9), similar to the above-mentionedexpression (2). $\begin{matrix}{{V\quad 19} = {{\frac{{V\quad 17} - {V\quad 18}}{2^{4}} \times N} + {V\quad 18}}} & (9)\end{matrix}$

Thus, the voltage division ratio x of the amplifier section 7A can bechanged by the electric signal Q.

As described above, according to the second embodiment of the presentinvention, the temperature of the heating element 1 (see FIG. 1) can beadjusted only by sending the electric signal Q for adjustment from thecomputer 22, so the adjustment can be made in a short time by a simplearrangement or device.

In addition, the voltage division ratio x of the amplifier section 7A isdecided by the ratio of the capacitance values, adjustment accuracy isnot influenced by the absolute accuracy of the capacitance values, soadjustment with a high degree of precision can be made even if highlyaccurate and expensive elements are not used.

Embodiment 3

Although in the above-mentioned first and second embodiments, theamplifier part 20 comprising the resistor array or the amplifier part20A comprising the capacitor array is used, an amplifier part 20Bcomprising an integrator may be used, as shown in FIG. 6.

FIG. 6 is a circuit configuration diagram that shows an amplifiersection 7B according to a third embodiment of the present invention,wherein the like parts or components as those described above (see FIGS.2 and 4) are identified by the same symbols or by the same symbols with“B” affixed to their ends, while omitting a detailed explanationthereof. Also, the circuit configuration of the sensor as a whole is asshown in FIG. 1.

In FIG. 6, the amplifier part 20B is composed of an integrator in theform of a CR integration circuit. A junction between a resistor R and acapacitor C, which together constitute the amplifier part 20B, isconnected to a non-inverting input terminal (+) of an operationalamplifier 24. Here, note that the integrator is not limited to the CRintegration circuit, but may be an other type of integrator. This issimilar in respective embodiments to be described later.

An amplification factor control part 23B includes an operationalamplifier 23 a connected to an input terminal 17 of the amplifiersection 7B, an operational amplifier 23 b connected to an input terminal18 of the amplifier section 7B, an operational amplifier 23 c connectedto an output terminal of the operational amplifier 23 a through aresistor R23, and a switch S0 inserted between an output terminal of theoperational amplifier 23 b and an input terminal of the operationalamplifier 23 c. Each of the operational amplifiers 23 a through 23 c hasan output terminal thereof connected to its own inverting input terminal(−).

The amplification factor control part 23B operates intermittentlyaccording to an electric signal Q (e.g., a clock signal of an arbitraryfrequency) from the computer 22, and controls the amplification factorof the amplifier part 20B comprising the integrator by changing the dutyratio of the opened and closed times of the switch S0.

In FIG. 6, assuming that the opened time (the duty ratio) of the switchS0 is represented by tx and the closed time of the switch S0 isrepresented by (1−tx), an output voltage V19 of the amplifier section 7Bis represented by the following expression (10).V19=−tx(V17−V18)+V18  (10)

As can be seen from expression (10), the voltage division ratio x of theamplifier section 7B is decided by the duty ratio tx of the opened andclosed times of the switch S0.

Thus, the voltage division ratio x of the amplifier section 7B can bechanged by the electric signal Q. Accordingly, neither circuit elementssuch as the resistor R and the capacitor C, which together constitutethe amplifier part 20B, the resistor R23 which constitutes theamplification factor control part 23B, etc., nor the absolute accuracyof the clock frequency for the amplification factor control part 23B isrequired, so voltage division with a high degree of precision can bedone as long as the relative accuracy of the opened and closed times ofthe switch S0 is kept.

As described above, according to the third embodiment of the presentinvention, the temperature of the heating element 1 can be adjusted onlyby sending the electric signal Q for adjustment from the computer 22, sothe adjustment can be made in a short time by a simple arrangement ordevice.

In addition, the voltage division ratio x of the amplifier section 7B isdecided by the ratio of the opened and closed times of the switch S0,and hence adjustment accuracy is not influenced by the accuracies of theresistors R, R23, the capacitor C and the clock frequency, so adjustmentwith a high degree of precision can be made.

Embodiment 4

Although in the above-mentioned first through third embodiments,reference has been made to the case where the amplification factor ofthe amplifier part 20, 20A or 20B is set to 1 or less, an amplifier part20C may be constructed such that its amplification factor can be set toa value equal to or larger than 1, as shown in FIG. 7.

FIG. 7 is a circuit configuration diagram that shows an amplifiersection 7C according to a fourth embodiment of the present invention,wherein the like parts or elements as those described above (see FIG. 2)are identified by the same symbols or by the same symbols with “C”affixed to their ends, while omitting a detailed explanation thereof.Also, the circuit configuration of the sensor as a whole is as shown inFIG. 1.

In FIG. 7, the amplifier part 20C includes, in addition to theabove-mentioned resistor array (see FIG. 2), operational amplifiers 25,26 and resistors R40, R50, and it is constructed in such a manner thatits amplification factor can be set to not only a value equal to or lessthan 1 but also a value equal to or larger than 1.

In this case, the above-mentioned resistor array has one end connectedto one input terminal 17 of the amplifier section 7C and an invertinginput terminals (−) of the operational amplifier 25, and the other endconnected to an output terminal 27 of the operational amplifier 25through an amplification factor control part 23 (switches S1 throughS4). Thus, the resistor array together with the operational amplifier 25constitutes an inverting amplifier.

A resistor 2R0 has one end thereof connected to the input terminal 17,and resistors 2R1 through 2R4 have one ends thereof connected to theinput terminal 17 or the output terminal 27 of the operational amplifier25 through the switches S1 through S4, respectively.

The amplifier section 7C has another input terminal 18 connected tonon-inverting input terminals (+) of the operational amplifiers 25, 26.The output terminal 27 of the operational amplifier 25 is connected toan inverting input terminal (−) of the operational amplifier 26 throughthe resistor R40, and the operational amplifier 26 has an outputterminal connected to an output terminal 19 of the amplifier section 7C,and at the same time connected to its own inverting input terminal (−)through the resistor R50.

FIG. 8 is a circuit configuration diagram that illustrates one exampleof the amplifier part 20C, wherein there is shown an equivalent circuitin the case where for example, the resistors 2R1, 2R2 are connected tothe input terminal 17 by means of the switches S1, S2, respectively, inFIG. 7, and the resistors 2R3, 2R4 are connected to the output terminal27 of the operational amplifier 25 by means of the switches S3, S4,respectively. In FIG. 8, individual resistance values or combinedresistance values are represented by “R” and “2R”.

At this time, an output voltage V27 at the output terminal 27 of theoperational amplifier 25 is represented by the following expression(11). $\begin{matrix}{{V\quad 27} = {{{- \frac{1}{3}}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}}} & (11)\end{matrix}$

In addition, from expression (11) above, an output voltage V19 of theamplifier section 7C is represented by the following expression (12).$\begin{matrix}\begin{matrix}{{V\quad 19} = {{- \left( {{V\quad 27} - {V\quad 18}} \right)} + {V\quad 18}}} \\{= {{\frac{1}{3}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}}}\end{matrix} & (12)\end{matrix}$

Here, note that the general formula of the output voltage V19 becomes,as shown by the following expression (13). $\begin{matrix}{{V\quad 19} = {{\frac{2^{4} - N}{N}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}}} & (13)\end{matrix}$

As can be seen from expression (13), the amplification factor of 1 ormore can be obtained depending on the setting of the digital value N.

In the example of FIG. 8, the digital value N is 1100 in binary number,which is 12 in decimal number (N=1100 (binary number)=12 (decimalnumber)), so the output voltage V19 becomes, as shown by the followingexpression (14). $\begin{matrix}{{V\quad 19} = {{{\frac{2^{4} - 12}{12}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}} = {{\frac{4}{12}\left( {{V\quad 17} - {V\quad 18}} \right)} + {V\quad 18}}}} & (14)\end{matrix}$

Accordingly, the above-mentioned expression (12) is obtained fromexpression (14) above.

Here, as shown in FIG. 9 (corresponding to FIG. 1), assuming that theamplification factor of the amplifier section 7C is represented by G,the output voltage V19 from the output terminal 19 of the amplifiersection 7C is represented by the following expression (15).V19=G(V17−V18)+V18=G·V17=G·V13  (15)

In expression (15) above, a voltage V13 at a junction 13 is representedas shown in the above-mentioned expression (4), so the expression (15)becomes as shown in the following expression (16). $\begin{matrix}{{V\quad 19} = {{G \cdot \frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}}V\quad 11}} & (16)\end{matrix}$

Here, an input voltage V15 to a non-inverting input terminal 15 of theoperational amplifier 8 is represented as shown in the above-mentionedexpression (5), and the input voltage V15 and the output voltage V19 ofthe amplifier section 7C become equal to each other, so the relation ofthe following expression (17) holds. $\begin{matrix}{\quad{{\frac{R5}{{R\quad 1} + {R5}}V\quad 11} = {G\quad\frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}}} & (17)\end{matrix}$

Accordingly, the resistance value R1 of the heating element 1 isrepresented by the following expression (18). $\begin{matrix}{{R\quad 1} = \frac{\left\{ {{R\quad 2} + {R\quad 3} + {\left( {1 - G} \right)R\quad 4}} \right\} R\quad 5}{{G \cdot R}\quad 4}} & (18)\end{matrix}$

As can be seen from expression (18) above, by changing the amplificationfactor G of the amplifier section 7C, the resistance value, i.e., thetemperature, of the heating element 1 can be adjusted. At this time, theamplification factor G is changed by the electric signal Q input to theamplification factor control part 23 (see FIG. 7).

As described above, according to the fourth embodiment of the presentinvention, the temperature of the heating element 1 can be adjusted onlyby sending the electric signal Q for adjustment from the computer 22, sothe adjustment can be made in a short time by a simple arrangement ordevice.

In addition, the amplification factor G (corresponding to the voltagedivision ratio x) of the amplifier section 7C is decided by the ratio ofthe resistance values of the resistors that constitute the resistorarray, and adjustment accuracy is not influenced by the absoluteaccuracies of the respective resistors, so adjustment with a high degreeof precision can be made.

Embodiment 5

Although in the above-mentioned first through fourth embodiments, noparticular reference has been made to the mounting structure of athermal flow sensor, the individual circuit elements may be arranged ona circuit board 29 and electrically connected to one another, as shownin FIG. 10.

FIG. 10 is an explanatory view that shows the circuit mountingarrangement of a thermal flow sensor according to a fifth embodiment ofthe present invention, wherein the like parts or elements as thosedescribed above (see FIGS. 1 and 2) are identified by the same symbolswhile omitting a detailed description thereof.

In FIG. 10, an IC 28 is mounted on the circuit board 29, and anamplifier part 20, a memory 21 and an amplification factor control part23 are integrated into the IC 28. In addition, resistors 3, 4, 5, anoperational amplifier 8 and a transistor 9, which together constitute abridge circuit, are mounted on the circuit board 29. A heating element 1and a fluid temperature detection element 2 are connected to the IC 28by wire bonding or the like. The amplifier part 20 integrated in the IC28 is not limited to a resistor array (see FIG. 2), but may beapplicable even if such a resistor array is replaced by any of acapacitor array (see FIG. 4), an integrator (see FIG. 6) or an invertingamplifier (see FIG. 7).

As described above, according to the fifth embodiment of the presentinvention, the amplifier part 20, the memory 21 and the amplificationfactor control part 23 are integrated into the IC 28, whereby theamplifier section 7 can be reduced in size.

Embodiment 6

Although in the above-mentioned fifth embodiment, the amplifier part 20is integrated into the IC 28 while giving priority to the size reductionof the circuit, a highly accurate and large-sized amplifier part 20D maybe mounted on a circuit board 29D outside of an IC 28D, as shown in FIG.11.

FIG. 11 is an explanatory view that shows the circuit mountingarrangement of a thermal flow sensor according to a sixth embodiment ofthe present invention, wherein the like parts or components as thosedescribed above (see FIG. 10) are identified by the same symbols or bythe same symbols with “D” affixed to their ends, while omitting adetailed explanation thereof.

In FIG. 11, an IC 28D is mounted on the circuit board 29D, and a memory21 and an amplification factor control part 23 are integrated into theIC 28D. In this case, the amplifier part 20D, being large in size andhigh in accuracy, is mounted on the circuit board 29D outside of the IC28D.

The amplifier part 20D mounted on the circuit board 29D is not limitedto the above-mentioned resistor array, but may be applicable even ifsuch a resistor array is replaced by any of a capacitor array, anintegrator or an inverting amplifier.

As described above, according to the sixth embodiment of the presentinvention, the amplifier part 20D is mounted on the circuit board 29Doutside of the IC 28D, so the amplifier part 29D, which is higher inaccuracy than the amplifier part 20 formed or built in the IC 28, can beused, thus making it possible to further improve adjustment accuracy.

Though such an operational effect is contrary or contradictory to thatof the above-mentioned fifth embodiment, either the fifth embodiment orthe sixth embodiment may be selected according to whether priority isgiven to accuracy improvement or size reduction.

Embodiment 7

Although in the above-mentioned sixth embodiment (see FIG. 11), noconsideration has been given to the dielectric strength of switches thatconstitute the amplification factor control part 23 inside the IC 28D,in consideration of the case where a resistive circuit for overvoltageprotection is needed (i.e., the dielectric strength of the switches inthe IC 28D is low), a switching circuit 30, which constitutes a partialfunction of an amplification factor control part 23E, may be providedbesides the amplification factor control part 23E (outside an IC 28E),for example as shown in FIG. 12.

FIG. 12 is a circuit configuration diagram that shows an amplifiersection 7E according to a seventh embodiment of the present invention,wherein the like parts or components as those described above (see FIGS.2 and 11) are identified by the same symbols or by the same symbols with“E” affixed to their ends, while omitting a detailed explanationthereof.

Here, note that the overall configuration of a thermal flow sensoraccording to the seventh embodiment of the present invention is as shownin FIG. 1, and the overall mounting arrangement thereof is as shown inFIG. 11.

In FIG. 12, a memory 21 and the amplification factor control part 23Eare integrated into the IC 28E. On the other hand, on a circuit boardoutside the IC 28E, there are mounted an amplifier part 20, a switchingcircuit 30 comprising semiconductor switches Sa through Sd with highdielectric resistance, and a resistive circuit 31 comprising resistorsRa through Rd to protect the switches S1 through S4 in the amplificationfactor control part 23E from overvoltage.

Switch control signals output from one ends of the individual switchesS1 through S4 in the amplification factor control part 23E are impressedto the corresponding semiconductor switches Sa through Sd, respectively,in the switching circuit 30 through the respective resistors Ra throughRd in the resistive circuit 31.

The switching circuit 30 constitutes a part of the amplification factorcontrol part 23E, and is driven to control the amplification factor ofthe amplifier part 20 by the switch control signals input from theamplification factor control part 23E through the resistive circuit 31.

In general, in case where only the amplifier part 20 is formed on thecircuit board outside of the IC 28E, it is necessary to connect betweencomponent parts from the amplification factor control part 23E in the IC28E to the amplifier part 20 outside of the IC 28 E by means of wiring.In addition, depending upon a method employed for manufacturing the IC28E, there might be in some cases the need to connect the resistivecircuit 31 to the switches S1 through S4 in the amplification factorcontrol part 23E in order to protect them from overvoltage.

At this time, since the resistive circuit 31 is formed in themanufacturing process of the IC 28E, there might arise the problem ofaccuracy such as the variation, the temperature characteristic, etc., ofthe resistive circuit. Accordingly, as shown in FIG. 12, the switchingcircuit 30 comprising the semiconductor switches Sa through Sd excellentin dielectric strength is mounted on the circuit board outside of the IC28E as a part of the function of the amplification factor control part23E, so that the switching circuit 30 is driven to control theamplification factor by means of the switch control signals from theamplification factor control part 23E in the IC 28E.

As described above, according to the seventh embodiment of the presentinvention, even in the case of need of the resistive circuit 31 forovervoltage protection due to the problem of dielectric strength in theswitches S1 through S4 in the amplification factor control part 23E inthe IC 28E, it is possible to adjust the temperature of the heatingelement 1 (see FIG. 1) to a high degree of precision without receivingthe influence of the variation or the temperature characteristic of theresistive circuit 31 for protection.

Embodiment 8

In the above-mentioned first through seventh embodiments, reference hasbeen made to the case where the present invention is applied to adirectly heated type thermal flow sensor in which a bridge circuit iscomposed of the heating element 1, the fluid temperature detectionelement 2 and the resistors 3 through 5, but the present invention mayalso be applied to an indirectly heated type flow sensor that includes,as shown in FIG. 13 for example, a temperature detection element 32 fordetecting the temperature of a heating element 1, a resistor 33connected in series to the heating element 1, and a constant voltagesource 34 connected to a bridge circuit which comprises the temperaturedetection element 32, a fluid temperature detection element 2 andresistors 3 through 5.

FIG. 13 is a circuit configuration diagram showing a thermal flow sensoraccording to an eighth embodiment of the present invention, wherein thelike parts or components as those described above (see FIG. 1) areidentified by the same symbols while omitting a detailed descriptionthereof.

In FIG. 13, a series circuit comprising a transistor 9, the heatingelement 1 and the resistor 33 is inserted between a power supply 10 andground, and a junction between the heating element 1 and the resistor 33is connected to an output terminal 14 of the thermal flow sensor.

In addition, a series circuit comprising the temperature detectionelement 32 and the resistor 5 and a series circuit comprising theresistor 3, the fluid temperature detection element 2 and the resistor 4are inserted in parallel to each other between the constant voltagesource 34 and ground.

A junction between the fluid temperature detection element 2 and theresistor 4 is connected to a non-inverting input terminal (+) of abuffer circuit 6, and a junction between the temperature detectionelement 32 and the resistor 5 is connected to a non-inverting inputterminal 15 of an operational amplifier 8. The operational amplifier 8has an output terminal thereof connected to a base terminal of thetransistor 9.

As shown in FIG. 13, the bridge circuit is composed of the temperaturedetection element 32 of the heating element 1, the fluid temperaturedetection element 2 and the resistors 3 through 5, and it is connectedto the constant voltage source 34. The temperature detection element 32is formed in the extreme vicinity of the heating element 1, so that thetemperature detected by the temperature detection element 32 comes toindicate a value substantially equal to an actual temperature of theheating element 1.

According to the circuit configuration of FIG. 13, the current suppliedto the heating element 1 is controlled so that the bridge circuit keepsan equilibrium state, as a result of which the electric power suppliedto the heating element 1 is controlled in such a manner that thetemperature of the heating element 1 detected by the temperaturedetection element 32 coincides with a predetermined control temperature.Accordingly, in the indirectly heated type flow sensor, by amplifying avoltage across the opposite ends of the resistor 4 by means of theamplifier section 7, operational effects similar to those obtained inthe above-mentioned individual first through seventh embodiments can beachieved. This is similar in respective embodiments to be describedlater.

As described above, according to the eighth embodiment of the presentinvention, the temperature of the heating element 1 can be adjusted onlyby sending an electric signal Q for adjustment (see FIG. 2) from anexternal computer to the amplifier section 7, so the adjustment can bemade in a short time by a simple arrangement or device.

In addition, since the voltage division ratio x of the amplifier section7 is decided by the ratio of the resistors in the amplifier section 7,adjustment accuracy is not influenced by the absolute accuracies of theresistors, and adjustment with a high degree of precision can be made.

Embodiment 9

In the above-mentioned first through eighth embodiments, one bridgecircuit corresponding to the one heating element 1 is used, but as shownin FIG. 14 for example, two bridge circuits corresponding to two heatingelement 1 a, 1 b may be arranged in parallel to each other, with anoutput terminal of each bridge circuit being connected to an outputterminal 14 of a thermal flow sensor through a differential amplifier35.

FIG. 14 is a circuit configuration diagram that shows a thermal flowsensor according to a ninth embodiment of the present invention, whereinthe like parts as those described above (see FIG. 1) are identified bythe same symbols or by the same symbols with “a” or “b” affixed to theirends, while omitting a detailed explanation thereof.

In FIG. 14, there is shown one example in a case where two bridgecircuits are arranged in parallel to each other, each bridge circuithaving the same configuration as that of the above-mentioned directlyheated type thermal flow sensor (FIG. 1). In FIG. 14, the heatingelement 1 a, a fluid temperature detection element 2 a and resistors 3 athrough 5 a together constitute a first bridge circuit, and a junction12 a between the heating element 1 a and the resistor 5 a, which is anoutput terminal of the first bridge circuit, is connected to anon-inverting input terminal (+) of the differential amplifier 35. Also,the heating element 1 b, a fluid temperature detection element 2 b andresistors 3 b through 5 b together constitute a second bridge circuit,and a junction 12 b between the heating element 1 b and the resistor 5b, which is an output terminal of the second bridge circuit, isconnected to an inverting input terminal (−) of the differentialamplifier 35.

The heating element 1 a and the heating element 1 b are formed insequence and in parallel with respect to the flow of fluid, and in caseof the fluid flow being downflow, the heating elements 1 a, 1 b arearranged in such a manner that the heating element 1 a in the firstbridge circuit is located at an upstream side of the heating element 1 bin the second bridge circuit. When the fluid flow is downflow, theheating element 1 a located at the upstream side is cooled more easilyor effectively than the heating element 1 b at the downstream side, sothe current supplied to the heating element 1 a of the first bridgecircuit becomes larger than the current supplied to the heating element1 b of the second bridge circuit. In this case, a voltage at thejunction 12 a (voltage V5 a across the opposite ends of the resistor 5a) output from the first bridge circuit becomes larger than a voltage atthe junction 12 b (voltage V5 b across the opposite ends of the resistor5 b).

The differential amplifier 35 calculates a voltage deviation (=V5 a−V5b) between the output voltage of the first bridge circuit (voltage V5 aat the junction 12 a) and the output voltage of the second bridgecircuit (voltage V5 b at the junction 12 b), and outputs it from itsoutput terminal 14 to the following circuit (not shown).

On the other hand, when the fluid flow is backflow, the output voltageof the first bridge circuit (voltage V5 a at the junction 12 a) becomessmaller than the output voltage of the second bridge circuit (voltage V5b at the junction 12 b), so the state of backflow can be detected.

In addition, similarly as stated above, the amplifier sections 7 a, 7 bamplify the voltages across the opposite ends of the resistors 4 a, 4 bin the first and second bridge circuits, respectively, and impress themto the inverting input terminals 16 a, 16 b of the operationalamplifiers 8 a, 8 b, respectively. As a result, the operationalamplifiers 8 a, 8 b control the transistors 9 a, 9 b, respectively,whereby the currents supplied to the heating elements 1 a, 1 b arecontrolled in an appropriate manner.

Thus, in a so-called double heater type thermal flow sensor, too, byamplifying the voltages across the opposite ends of the resistors 4 a, 4b in the individual bridge circuits by means of the amplifier sections 7a, 7 b, respectively, and by controlling the transistors 9 a, 9 bthrough the individual operational amplifiers 8 a, 8 b, respectively, itis possible to achieve operational effects similar to theabove-mentioned ones. This is similar in respective embodiments to bedescribed later.

As described above, according to the ninth embodiment of the presentinvention, the temperatures of the heating elements 1 a, 1 b can beadjusted only by sending electric signals for adjustment from anexternal computer to the amplifier sections 7 a, 7 b, so the adjustmentcan be made in a short time by a simple arrangement or device. Inaddition, since the voltage division ratios x of the amplifier sections7 a, 7 b are decided by the ratios of the resistors in the amplifiersections 7 a, 7 b, respectively, adjustment accuracy is not influencedby the absolute accuracies of the resistors, and adjustment with a highdegree of precision can be made.

Embodiment 10

In the above-mentioned ninth embodiment, the two heating elements 1 a, 1b are arranged at the upstream and downstream sides, respectively, withrespect to the flow of fluid thereby to form the two bridge circuitscorresponding to the heating elements 1 a, 1 b, respectively, but asshown in FIG. 15 for example, temperature detection elements 36, 37 maybe arranged at the upstream and downstream sides of one heating element1, and a second bridge circuit is composed of the temperature detectionelements 36, 37 and resistors 38, 39, with two junctions of the secondbridge circuit being connected to an output terminal 14 of a thermalflow sensor through a differential amplifier 35.

FIG. 15 is a circuit configuration diagram that shows a thermal flowsensor according to a tenth embodiment of the present invention, whereinthe like parts or components as those described above (see FIGS. 1 and14) are identified by the same symbols while omitting a detaileddescription thereof.

In FIG. 15, the temperature detection elements 36, 37 for detecting thetemperature of the heating element 1, which is similar to theabove-mentioned directly heated type thermal flow sensor (see FIG. 1),are formed or arranged at the upstream and downstream sides,respectively, of the heating element 1, whereby the second bridgecircuit is composed of the temperature detection elements 36, 37 and theresistors 38, 39.

Also, in FIG. 15, a junction between the temperature detection element36 and the resistor 39 is connected to an inverting input terminal (−)of the differential amplifier 35, and a junction between the temperaturedetection element 37 and the resistor 38 is connected to a non-invertinginput terminal (+) of the differential amplifier 35.

In addition, when the fluid flow is downflow, the temperature detectionelement 36 is arranged to locate at the upstream side of the heatingelement 1, and the temperature detection element 37 is arranged tolocate at the downstream side of the heating element 1. In this case,the temperature detection element 36 located at the upstream side iscooled more easily or effectively than the temperature detection element37 located at the downstream side, so the temperature of the temperaturedetection element 36 becomes lower than the temperature of thetemperature detection element 37.

In the second bridge circuit, the temperature detected by thetemperature detection element 36 is output as a voltage V39 across theopposite ends of the resistor 39, and the temperature detected by thetemperature detection element 37 is output as a voltage V38 across theopposite ends of the resistor 38.

The differential amplifier 35 calculates a voltage deviation (=V38−V39)between the voltage V38 across the opposite ends of the resistor 38 andthe voltage V39 across the opposite ends of the resistor 39, and outputsit from its output terminal 14 to the following circuit. The outputvoltage of the differential amplifier 35 indicates a temperaturedeviation between the temperature detected by the temperature detectionelement 36 and the temperature detected by the temperature detectionelement 37.

On the other hand, when the fluid flow is backflow, the temperaturedetected by the temperature detection element 36 becomes higher than thetemperature detected by the temperature detection element 37, so thestate of backflow can be detected.

In addition, similarly as stated above, the amplifier section 7amplifies the voltage across the opposite ends of the resistor 4 in thefirst bridge circuit, and impress it to the inverting input terminal 16of the operational amplifier 8, whereby the operational amplifier 8controls the transistor 9 thereby to adjust the current supplied to theheating element 1 in an appropriate manner.

Thus, in a so-called temperature difference detection type thermal flowsensor, too, operational effects similar to the above-mentioned ones canbe obtained by amplifying the voltage across the opposite ends of theresistor 4 in the first bridge circuit by means of the amplifier section7. This is similar in respective embodiments to be described later.

As described above, according to the tenth embodiment of the presentinvention, the temperature of the heating element 1 can be adjusted onlyby sending an electric signal Q for adjustment (see FIG. 2) from anexternal computer, so the adjustment can be made in a short time by asimple arrangement or device. In addition, since the voltage divisionratio x of the amplifier section 7 is decided by the ratio of theresistors in the amplifier section 7, adjustment accuracy is notinfluenced by the absolute accuracies of the resistors, and adjustmentwith a high degree of precision can be made.

Embodiment 11

Although in the above-mentioned first through tenth embodiments, onlyone heating element is arranged in one bridge circuit, two heatingelements 1A, 1B may be arranged in series with each other in one bridgecircuit, and similarly, two fluid temperature detection elements 2A, 2Bmay be arranged in series with each other, as shown in FIG. 16 forexample.

FIG. 16 is a circuit configuration diagram that shows a thermal flowsensor according to an eleventh embodiment of the present invention,wherein the like parts or components as those described above areidentified by the same symbols or by the same symbols with “A” or “B”affixed to their ends, while omitting a detailed explanation thereof.

In the bridge circuit shown in FIG. 16, a junction 13A between the fluidtemperature detection elements 2A, 2B connected in series with eachother is connected to an input terminal 17 of an amplifier section 7through a buffer circuit 6. Also, a junction 12A between the heatingelements 1A, 1B connected in series with each other is connected to anon-inverting input terminal 15 of an operational amplifier 8. The fluidtemperature detection element 2B has one end thereof at a ground sideconnected to one end of a resistor 4, which has one terminal at a groundside connected to an input terminal 18 of the amplifier section 7. As aresult, the amplifier section 7 amplifies a voltage across the oppositeends of a series circuit comprising the fluid temperature detectionelement 2B and the resistor 4.

Similarly as stated above, when the amplifier section 7 divides an inputvoltage into a ratio of “x:1−x”, and outputs the thus divided voltage,an input voltage V16 to an inverting input terminal 16 of theoperational amplifier 8 is represented by using a voltage V13 a at thejunction 13A between the fluid temperature detection elements 2A, 2B, asshown by the following expression (19).V16=(1−x)V13a  (19)

In addition, the voltage V13 a is represented by using a voltage V11 ata junction 11 between the heating element 1A and a resistor 3, as shownby the following expression (20). $\begin{matrix}{{V\quad 13a} = {\frac{{R\quad 2b} + {R\quad 4}}{{R\quad 2a} + {R\quad 2b} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (20)\end{matrix}$where R2 a and R2 b are the individual resistance values of the fluidtemperature detection element 2A, 2B, respectively.

On the other hand, an input voltage V15 to the non-inverting inputterminal 15 of the operational amplifier 8 is represented by thefollowing expression (21). $\begin{matrix}{{V\quad 15} = {\frac{{R\quad 1b} + {R\quad 5}}{{R\quad 1a} + {R\quad 1b} + {R\quad 5}}V\quad 11}} & (21)\end{matrix}$where R1 a and R1 b are the individual resistance values of the heatingelement 1A, 1B, respectively.

Here, if the input terminals 15, 16 of the operational amplifier 8 arevirtually short-circuited with each other, the input voltages V15, V16to the input terminals 15, 16 become equal to each other (V15=V16), andhence the above expression (21) is represented by the followingexpression (22). $\begin{matrix}{{\frac{{R\quad 1b} + {R\quad 5}}{{R\quad 1a} + {R\quad 1b} + {R\quad 5}}V\quad 11} = {\left( {1 - x} \right)\frac{{R\quad 2b} + {R\quad 4}}{{R\quad 2a} + {R\quad 2b} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (22)\end{matrix}$

Accordingly, the resistance value R1 a of the heating element 1A isrepresented by the following expression (23) in association with theresistance value R1 b of the heating element 1B. $\begin{matrix}{{R\quad 1a} = \frac{\left\{ {{R\quad 3} + {R\quad 2a} + {x\left( {{R\quad 2b} + {R\quad 4}} \right)}} \right\}\left( {{R\quad 1b} + {R\quad 5}} \right)}{\left( {1 - x} \right)\left( {{R\quad 2b} + {R\quad 4}} \right)}} & (23)\end{matrix}$

As can be seen from expression (23), by changing the voltage divisionratio x of the amplifier section 7, the resistance value, i.e., thetemperature, of the heating element 1A can be adjusted. Also, since theresistance value R1 b of the heating element 1B exists at the right-handside of expression (23), the temperature of the heating element 1A isvaried by the flow rate of fluid.

Thus, the thermal flow sensor using the serially connected heatingelements 1A, 1B and the serially connected fluid temperature detectionelements 2A, 2B, though well-known (see, for example, Japanese patentapplication laid-open No. 2002-5717), is a system that can achieve anadvantageous effect of improving the temperature characteristic of thesensor. In case where the present invention is applied to such a kind ofthermal flow sensor, operational effects similar to the above-mentionedones can be obtained by adjusting the amplification factor by means ofthe amplifier section 7. This is similar in respective embodiments to bedescribed later.

As described above, according to the eleventh embodiment of the presentinvention, the temperatures of the heating elements 1A, 1B can beadjusted only by sending an electric signal Q for adjustment (see FIG.2) from an external computer, so the adjustment can be made in a shorttime by a simple arrangement or device.

In addition, since the voltage division ratio x of the amplifier section7 is decided by the ratio of the resistance values of resistors,adjustment accuracy is not influenced by the absolute accuracies of theresistors, and adjustment with a high degree of precision can be made.

Embodiment 12

Although in the above-mentioned eleventh embodiment, a voltage acrossthe opposite ends of the series circuit of the fluid temperaturedetection element 2B and the resistor 4 is impressed to the amplifiersection 7, a voltage across the opposite ends of the fluid temperaturedetection element 2B may instead be impressed to the amplifier section7, as shown in FIG. 17, for example.

FIG. 17 is a circuit configuration diagram that shows a thermal flowsensor according to a twelfth embodiment of the present invention,wherein the like parts or components as those described above (see FIG.16) are identified by the same symbols or by the same symbols with “A”or “B” affixed to their ends, while omitting a detailed explanationthereof.

In FIG. 17, the voltage across the opposite ends of the fluidtemperature detection element 2B is impressed to the amplifier section 7through buffer circuits 6A, 6B connected to the individual ends thereof,respectively.

Here, similarly as stated above, when the amplifier section 7 divides aninput voltage into a ratio of “x:1−x”, and outputs the thus dividedvoltage, an output voltage V19 of the amplifier section 7 is representedby the following expression (24). $\begin{matrix}{{V\quad 19} = \frac{{\left( {1 - x} \right)R\quad 2b} + {R\quad 4}}{{R\quad 2a} + {R\quad 2b} + {R\quad 3} + {R\quad 4}}} & (24)\end{matrix}$

The output voltage V19 of the amplifier section 7 is impressed to aninverting input terminal 16 of an operational amplifier 8. On the otherhand, a voltage V15 (voltage at a junction 12A between heating elements1A, 1B), being represented by the above-mentioned expression (21), isimpressed to a non-inverting input terminal 15 of the operationalamplifier 8.

Accordingly, a resistance value R1 a of the heating element 1A isrepresented by the following expression (25) in association with aresistance value R1 b of the heating element 1B. $\begin{matrix}{{R\quad 1a} = \frac{\left( {{R\quad 3} + {R\quad 2a} + {{xR}\quad 2b}} \right)\left( {{R\quad 5} + {R\quad 1b}} \right)}{{\left( {1 - x} \right)R\quad 2b} + {R\quad 4}}} & (25)\end{matrix}$

As can be seen from expression (25), by changing the voltage divisionratio x of the amplifier section 7, the resistance value, i.e., thetemperature, of the heating element 1A can be adjusted.

Thus, operational effects similar to the above-mentioned ones areobtained even with the circuit configuration that amplifies the voltageacross the opposite ends of the fluid temperature detection element 2B.This is similar in respective embodiments to be described later.

As described above, according to the twelfth embodiment of the presentinvention, the temperatures of the heating elements 1A, 1B can beadjusted only by sending an electric signal Q for adjustment (see FIG.2) from an external computer, so the adjustment can be made in a shorttime by a simple arrangement or device.

In addition, since the voltage division ratio x of the amplifier section7 is decided by the ratio of the resistance values of resistors,adjustment accuracy is not influenced by the absolute accuracies of theresistors, and adjustment with a high degree of precision can be made.

Embodiment 13

Although in the above-mentioned first through twelfth embodiments, oneamplifier section is used for one bridge circuit, two amplifier sections7F, 7G may be used for one bridge circuit, as shown in FIG. 18 forexample.

FIG. 18 is a circuit configuration diagram that shows a thermal flowsensor according to a thirteenth embodiment of the present invention,wherein the like parts or components as those described above (seeFIG. 1) are identified by the same symbols or by the same symbols with“C” through “G” affixed to their ends, while omitting a detailedexplanation thereof.

In FIG. 18, two amplifier sections 7F, 7G are arranged for the bridgecircuit. The amplifier section 7F constitutes a DA converter similar tothe above-mentioned one (see FIG. 2), and includes, for example, anamplifier part 20 in the form of a resistor array, and an amplificationfactor control part 23 in the form of switches that can be electricallycontrolled. Also, the amplifier section 7G is constructed similar to theamplifier section 7F.

A junction between a fluid temperature detection element 2 and aresistor 4 is connected to an input terminal 17F of the amplifiersection 7F through a buffer circuit 6C, and the resistor 4 has aterminal at a ground side connected to an input terminal 18F of theamplifier section 7F. The amplifier section 7F has an output terminal19F connected to one end (an end that is not connected to the heatingelement 1) of a resistor 5 through a buffer circuit 6D. A voltage acrossthe opposite ends of the resistor 4 is impressed to the amplifiersection 7F through the buffer circuit 6C, and the amplifier section 7Fdivides an input voltage (amplifies it at a ratio of 1 or less) andoutputs the thus divided voltage.

On the other hand, a junction 11 between a transistor 9 and a resistor 3is connected to an input terminal 17G of the amplifier section 7Gthrough a buffer circuit 6E. A junction between the resistor 3 and thefluid temperature detection element 2 is connected to an input terminal18G of the amplifier section 7G through a buffer circuit 6F, and theamplifier section 7G has an output terminal 19G connected to one end ofthe heating element 1 (an end that is not connected to the resistor 5)through a buffer circuit 6G. A voltage across the opposite ends of theresistor 3 is impressed to the amplifier section 7G through the buffercircuits 6E, 6F, and the amplifier section 7G divides the input voltagein an appropriate manner and outputs it.

As stated above, the fluid temperature detection element 2 is formed orarranged at a location at which it does not receive the influence ofheat from the heating element 1, and constitutes a bridge circuittogether with the heating element 1 and the resistors 3, 4, 5.

A voltage at a junction 13 between the fluid temperature detectionelement 2 and the resistor 4 is input to an inverting input terminal 16of an operational amplifier 8, and a voltage at a junction 12 betweenthe heating element 1 and the resistor 5 is impressed to a non-invertinginput terminal 15 of the operational amplifier 8. The operationalamplifier 8 has its output terminal connected to a base terminal of thetransistor 9, which has an emitter connected to one end of the resistor3.

In FIG. 18, when the amplifier section 7F divides the input voltage intoa ratio of “x:1−x” and outputs the thus divided voltage, and when theamplifier section 7G divides the input voltage into a ratio of “1−y:y”and outputs the thus divided voltage, an output voltage V19 f of theamplifier 7F at a ground side is represented by using the individualresistance values R2 through R4 of the resistors 2 through 4 and thevoltage V11 at the junction 11, as shown by the following expression(26). $\begin{matrix}{{V\quad 19f} = {\left( {1 - x} \right)\frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (26)\end{matrix}$

Also, an output voltage V19 g of the amplifier section 7G at the powersupply 10 side is represented by the following expression (27).$\begin{matrix}{{V\quad 19g} = {\frac{{R\quad 2} + {{yR}\quad 3} + {R\quad 4}}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (27)\end{matrix}$

Accordingly, an input voltage V15 to the non-inverting input terminal 15of the operational amplifier 8 is represented by the followingexpression (28). $\begin{matrix}\begin{matrix}{{V\quad 15} = {{V\quad 19f} + {\frac{R\quad 5}{{R\quad 1} + {R\quad 5}}\left( {{V\quad 19g} - {V\quad 19f}} \right)}}} \\{= {{\frac{R\quad 1}{{R\quad 1} + {R\quad 5}}V\quad 19f} + {\frac{R\quad 5}{{R\quad 1} + {R\quad 5}}V\quad 19g}}} \\{= {{\left( {1 - x} \right)\frac{R\quad 1}{{R\quad 1} + {R\quad 5}}\frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11} +}} \\{\frac{R\quad 5}{{R\quad 1} + {R\quad 5}}\frac{{R\quad 2} + {{yR}\quad 3} + {R\quad 4}}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}\end{matrix} & (28)\end{matrix}$

In addition, an input voltage V16 to the inverting input terminal 16 ofthe operational amplifier 8 is represented by the following expression(29). $\begin{matrix}{{V\quad 16} = {\frac{R\quad 4}{{R\quad 2} + {R\quad 3} + {R\quad 4}}V\quad 11}} & (29)\end{matrix}$

Here, from V15=V16, a resistance value R1 of the heating element 1 isrepresented, as shown by the following expression (30). $\begin{matrix}{{R\quad 1} = \frac{\left( {{R\quad 2} + {{yR}\quad 3}} \right)R\quad 5}{{xR}\quad 4}} & (30)\end{matrix}$

On the other hand, the resistance value R2 of the fluid temperaturedetection element 2 is changed in accordance with the temperaturethereof, so it is replaced as shown by the following expression (31).R2=R20(1+α₂ T ₂)  (31)where R20 is the resistance value of the fluid temperature detectionelement 2 at a temperature of 0° C.; α2 is the temperature coefficientof resistance of the fluid temperature detection element 2; and T2 isthe temperature of the fluid temperature detection element 2.

When expression (31) is assigned to expression (30) and the result isrearranged, the resistance value R1 of the heating element 1 isrepresented by the following expression (32). $\begin{matrix}{{R\quad 1} = {\frac{\left( {{R\quad 20} + {{yR}\quad 3}} \right)R\quad 5}{{XR}\quad 4}\left( {1 + {\frac{R\quad 20}{{R\quad 20} + {{yR}\quad 3}}\alpha_{2}T_{2}}} \right)}} & (32)\end{matrix}$

In expression (32) above, the term R20/(R20+yR3)×α2 in the right-handside parentheses represents the temperature coefficient of theresistance value R1, which can be adjusted by a voltage division ratioy. Also, the resistance value R1 can be adjusted by the voltage divisionratio x. Accordingly, as can be seen from expression (32), theresistance value R1 and the temperature coefficient can be adjustedindependently from each other.

In the thermal flow sensor, it is necessary to adjust the resistancevalue R1 and the temperature coefficient of the heating element 1independently from each other for adjustment of the temperaturecharacteristic of the sensor, but according to the thirteenthembodiment, the present invention can be applied to such a case.

As described above, according to the thirteenth embodiment of thepresent invention, the temperature of the heating element 1 can beadjusted only by sending an electric signal for adjustment from anexternal computer, so the adjustment can be made in a short time by asimple arrangement or device.

In addition, since the voltage division ratio x of the amplifier section7 is decided by the ratio of the resistance values of resistors,adjustment accuracy is not influenced by the absolute accuracies of theresistors, and adjustment with a high degree of precision can be made.

Further, since the resistance value R1 and the temperature coefficientof the heating element 1 can be adjusted independently from each other,the temperature characteristic of the sensor can be adjusted by using asimple device and circuit.

Embodiment 14

Although in the above-mentioned thirteenth embodiment, each one of theresistors 3, 4 is connected in series to the opposite ends,respectively, of the fluid temperature detection element 2 (i.e., oneresistor for each end of the element), each two of resistors 3A, 3B and4A, 4B may be connected to the opposite ends, respectively, of the fluidtemperature detection element 2, as shown in FIG. 19 for example.

FIG. 19 is a circuit configuration diagram that shows a thermal flowsensor according to a fourteenth embodiment of the present invention,wherein the like parts or components as those described above (see FIG.18) are identified by the same symbols or by the same symbols with “A”or “B” affixed to their ends, while omitting a detailed explanationthereof.

In FIG. 19, each two of resistors 3A, 3B, and 4A, 4B are connected tothe opposite ends, respectively, of the fluid temperature detectionelement 2. Specifically, in this case, the above-mentioned resistor 3 isdivided into the resistor 3A and the resistor 3B, and theabove-mentioned resistor 4 is divided into the resistor 4A and theresistor 4B. A voltage across the opposite ends of the resistor 3B isdivided into a ratio of “1−w:w” by means of an amplifier section 7G, anda voltage across the opposite ends of the resistor 4B is divided into aratio of “z:1−z” by means of an amplifier section 7F.

Accordingly, the resistance value R1 of the heating element 1 isrepresented by the following expression (33). $\begin{matrix}{{R\quad 1} = \frac{\left( {{R\quad 2} + {R\quad 3a} + {{wR}\quad 3b}} \right)R\quad 5}{{R\quad 4a} + {{zR}\quad 4b}}} & (33)\end{matrix}$

When expression (33) above is compared with the aforementionedexpression (30), the following expressions (34), (35) hold.xR4=R4a+zR4b  (34)yR3=R3a+wR3b  (35)

The above expressions (34), (35) are formulated in such a manner thatadjustment terms (xR4, yR3) of the above-mentioned thirteenth embodimentare divided into fixed components (R4 a, R3 a) and variable components(zR4 b, wR3 b), respectively.

The resistance values R3 a, R4 a of the individual resistors 3A, 3B takeminimum values of yR3, xR4, respectively, and the resistance values R3b, R4 b of the respective resistors 3B, 4B take the values of the changewidths of yR3, xR4, respectively. As a result, only the adjustmentcomponents (corresponding to the resistor 3B, 4B) of the aforementionedresistors R3, R4 can be voltage divided.

As described above, according to the fourteenth embodiment of thepresent invention, the temperature of the heating element 1 can beadjusted only by sending an electric signal for adjustment from anexternal computer, so the adjustment can be made in a short time by asimple arrangement or device.

In addition, since the voltage division ratios w, z are decided by theratios of the resistance values of resistors, adjustment accuracy is notinfluenced by the absolute accuracies of the resistors, and adjustmentwith a high degree of precision can be made.

Moreover, since the resistance value and the temperature coefficient ofthe heating element 1 can be adjusted independently from each other, thetemperature characteristic of the sensor can be adjusted by using asimple device and circuit.

Further, since only the adjustment component is voltage divided andadjusted, high adjustment accuracy can be obtained even with lowresolution.

Embodiment 15

Although in the above-mentioned thirteenth and fourteenth embodiments,two amplifier sections 7F, 7G are arranged in a bridge circuit, twoamplifier sections 7, 7H may instead be arranged outside of a bridgecircuit, as shown in FIG. 20 for example.

FIG. 20 is a circuit configuration diagram that shows a thermal flowsensor according to a fifteenth embodiment of the present invention,wherein the like parts or components as those described above (seeFIG. 1) are identified by the same symbols or by the same symbols with“H” through “J” affixed to their ends, while omitting a detailedexplanation thereof.

In FIG. 20, the bridge circuit includes, similar to the above-mentionedfirst embodiment, a heating element 1, a fluid temperature detectionelement 2 that is formed or arranged at a location free from theinfluence of heat from the heating element 1, and a plurality ofresistors 3, 4, 5. A voltage across the opposite ends of the resistor 4is impressed to the amplifier section 7 through a buffer circuit 6. Theamplifier section 7 has an output terminal 19 connected to an invertinginput terminal 16 of an operational amplifier 8. The amplifier section 7constitutes a DA converter similar to the above-mentioned one (FIG. 2),and includes an amplifier part 20 in the form of a resistor array, andan amplification factor control part 23 in the form of switches that canbe electrically controlled. The amplifier section 7 serves to divide aninput voltage (amplify it at a ratio of 1 or less) and output the thusdivided voltage.

A voltage across the opposite ends of the heating element 1 is impressedto the amplifier section 7H through buffer circuits 6H, 6J. Theamplifier section 7H has a configuration similar to the amplifiersection 7, and serves to divide and output an input voltage. Theamplifier section 7H has an output terminal 19H connected to anon-inverting input terminal 15 of the operational amplifier 8. Theoperational amplifier 8 has its output terminal connected to a baseterminal of a transistor 9, which has an emitter connected to a junction11 between the heating element 1 and the resistor 3.

As shown in FIG. 20, when the amplifier section 7 divides the inputvoltage into a ratio of “x:1−x” and outputs the thus divided voltage,and when the amplifier section 7H divides the input voltage into a ratioof “1−y:y” and outputs the thus divided voltage, the resistance value R1of the heating element 1 is represented, as shown by the followingexpression (36). $\begin{matrix}{{R\quad 1} = \frac{\left( {{R\quad 2} + {R\quad 3} + {{xR}\quad 4}} \right)R\quad 5}{{\left( {1 - x} \right)R\quad 4} - {y\left( {{R\quad 2} + {R\quad 3} + {R\quad 4}} \right)}}} & (36)\end{matrix}$

As can be seen from expression (36), by changing the voltage divisionratios x, y of the amplifier sections 7, 7H, the resistance value, i.e.,the temperature, of the heating element 1 can be adjusted.

As described above, according to the fifteenth embodiment of the presentinvention, the temperature of the heating element 1 can be adjusted onlyby sending an electric signal for adjustment from an external computer,so the adjustment can be made in a short time by a simple arrangement ordevice.

In addition, since the voltage division ratios x, y of the amplifiersections 7, 7H are decided by the ratios of the resistance values ofresistors, adjustment accuracy is not influenced by the absoluteaccuracies of the resistors, and adjustment with a high degree ofprecision can be made.

Embodiment 16

Although in the above-mentioned fifteenth embodiment, the amplificationfactor of each of the amplifier sections 7, 7H is set to a value equalto or less than 1, amplification factors G1, G2 of individual amplifiersections 7′, 7H′ may be set to values equal to or larger than 1,respectively, as shown in FIG. 21 for example.

FIG. 21 is a circuit configuration diagram that shows a thermal flowsensor according to a sixteenth embodiment of the present invention,wherein the like parts or components as those described above (see FIG.20) are identified by the same symbols or by the same symbols with “′”affixed to their ends, while omitting a detailed explanation thereof.

In FIG. 21, the amplifier sections 7′, 7H′ are configured in such amanner that their amplification factors G1, G2 are equal to or largerthan 1. In this case, the resistance value R1 of the heating element 1is represented by using the amplification factor G1 of the amplifiersection 7′ and the amplification factor G2 of the amplifier section 7H′,as shown by the following expression (37). $\begin{matrix}{{R\quad 1} = \frac{\left( {{R\quad 2} + {R\quad 3} + {R\quad 4} - {G\quad 2R\quad 4}} \right)R\quad 5}{{G\quad 2R\quad 4} - {G\quad 1\left( {{R\quad 2} + {R\quad 3} + {R\quad 4}} \right)}}} & (37)\end{matrix}$

As can be seen from expression (37), by changing the amplificationfactors G1, G2 of the amplifier sections 7′, 7H′, the resistance value,i.e., the temperature, of the heating element 1 can be adjusted.Accordingly, the temperature of the heating element 1 can be adjustedonly by sending an electric signal for adjustment from an externalcomputer, so the adjustment can be made in a short time by a simplearrangement or device. In addition, the amplification factors G1, G2 ofthe individual amplifier sections 7′, 7H′ are decided by the ratios ofthe resistance values of resistors, adjustment accuracy is notinfluenced by the absolute accuracies of the resistors, and adjustmentwith a high degree of precision can be made.

As described in the foregoing, according to the present invention,similarly as stated above, by changing the amplification factors of theamplifier sections for each of the thermal flow sensors with individualvariations by means of an electric signal, it is possible to adjust thetemperature of the heating element 1 to a predetermined controltemperature, so processes and apparatuses for mechanical processing suchas soldering, laser trimming, etc., become unnecessary, and adjustmentcan be made in a short time with a simple device.

In addition, adjustment can be made while monitoring the temperature ofthe heating element 1 or an index indicating the temperature, so anadjustment process or step becomes simple.

Further, each of the amplification factors (G1, G2) is decided by theratio of a resistor array (or a capacitor array, or the opened andclosed times of the switches, etc.), so the absolute accuracies ofindividual circuit elements are not required, and hence adjustment witha high degree of precision can be made even if highly accurate elementsare not used.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modifications within the spirit and scope of theappended claims.

1. A thermal flow sensor in which a bridge circuit is composed of atleast one heating element, a fluid temperature detection element, and aplurality of resistors; said heating element and said fluid temperaturedetection element are arranged in fluid, and said fluid temperaturedetection element is arranged at a location free from the influence ofheat from said heating element; and a flow rate of said fluid isdetected by using the fact that an amount of heat transmitted from saidheating element to said fluid in a state where said heating element isalways held at a control temperature higher by a predetermined valuethan the temperature of said fluid detected by said fluid temperaturedetection element depends on the flow rate of said fluid; said thermalflow sensor comprising: an amplifier section that amplifies a voltageacross opposite ends of at least one of said plurality of resistors; acurrent control section that is controlled based on an output voltage ofsaid amplifier section; and an output terminal that is connected to oneend of said heating element, which is controlled to be energized throughsaid current control section, for outputting a detection resultcorresponding to the flow rate of said fluid; wherein said amplifiersection comprises an amplifier part that amplifies an input signal tosaid amplifier section, and an amplification factor control part thatcontrols an amplification factor of said amplifier part; saidamplification factor control part changes the amplification factor ofsaid amplifier part by means of an electric signal so that thetemperature of said heating element is adjusted to said controltemperature.
 2. The thermal flow sensor as set forth in claim 1, whereinsaid amplifier part and said amplification factor control part areintegrated into an IC.
 3. The thermal flow sensor as set forth inclaim
 1. wherein said amplification factor control part is integrated inan IC; and said amplifier part is formed on a circuit board connected tosaid IC.
 4. The thermal flow sensor as set forth in claim 1, whereinsaid amplifier part is composed of a resistor array; and saidamplification factor control part is composed of switches.
 5. Thethermal flow sensor as set forth in claim 1, wherein said amplifier partis composed of a capacitor array that includes a plurality ofcapacitors, and a switch array that is connected to the individualcapacitors of said capacitor array and operates at an arbitrary clockfrequency; and said amplification factor control part is composed ofswitches.
 6. The thermal flow sensor as set forth in claim 1, whereinsaid amplifier part is composed of an integrator; and said amplificationfactor control part is composed of switches.
 7. The thermal flow sensoras set forth in claim 5, wherein said amplification factor control partis composed of semiconductor switches connected to an IC.
 8. A thermalflow sensor in which a bridge circuit is composed of a temperaturedetection element arranged in the vicinity of at least one heatingelement, a fluid temperature detection element, and a plurality ofresistors; said heating element and said fluid temperature detectionelement are arranged in fluid, and said fluid temperature detectionelement is arranged at a location free from the influence of heat fromsaid heating element; and a flow rate of said fluid is detected by usingthe fact that an amount of heat transmitted from said heating element tosaid fluid in a state where said heating element is always held at acontrol temperature higher by a predetermined value than the temperatureof said fluid detected by said fluid temperature detection elementdepends on the flow rate of said fluid; said thermal flow sensorcomprising: an amplifier section that amplifies a voltage acrossopposite ends of at least one of said plurality of resistors; a currentcontrol section that is controlled based on an output voltage of saidamplifier section; and an output terminal that is connected to one endof said heating element, which is controlled to be energized throughsaid current control section, for outputting a detection resultcorresponding to the flow rate of said fluid; wherein said amplifiersection comprises an amplifier part that amplifies an input signal tosaid amplifier section, and an amplification factor control part thatcontrols an amplification factor of said amplifier part; saidamplification factor control part changes the amplification factor ofsaid amplifier part by means of an electric signal so that thetemperature of said heating element is adjusted to said controltemperature.
 9. A thermal flow sensor in which a first bridge circuit iscomposed of a first heating element arranged at an upstream side withrespect to a forward flow of fluid, a first fluid temperature detectionelement arranged at a location free from the influence of heat from saidfirst heating element, and a plurality of first resistors; a secondbridge circuit is composed of a second heating element arranged at adownstream side with respect to a forward flow of fluid, a second fluidtemperature detection element arranged at a location free from theinfluence of heat from said second heating element, and a plurality ofsecond resistors; and a flow rate of said fluid is detected by using thefact that an amount of heat transmitted from said first and secondheating elements to said fluid in a state where said first and secondheating elements are always held at a control temperature higher by apredetermined value than the temperature of said fluid detected by saidfirst and second fluid temperature detection elements depends on theflow rate of said fluid; said thermal flow sensor comprising: a firstamplifier section that amplifies a voltage across opposite ends of atleast one of said plurality of first resistors; a first current controlsection that is controlled based on an output voltage of said firstamplifier section; a first junction comprising one end of said firstheating element which is controlled to be energized through said firstcurrent control section; a second amplifier section that amplifies avoltage across opposite ends of at least one of said plurality of secondresistors; a second current control section that is controlled based onan output voltage of said second amplifier section; a second junctioncomprising one end of said second heating element which is controlled tobe energized through said second current control section; and an outputterminal that is connected to said first and second junctions foroutputting a detection result corresponding to the flow rate of saidfluid; wherein each of said first and second amplifier sectionscomprises an amplifier part that amplifies an input signal to saidamplifier sections, and an amplification factor control part thatcontrols an amplification factor of said amplifier part; said first andsecond amplification factor control parts in said first and secondamplifier sections change the amplification factors of said amplifierparts in said first and second amplifier sections, respectively, bymeans of an electric signal so that the temperatures of said first andsecond heating elements are adjusted to said control temperature.
 10. Athermal flow sensor in which a first bridge circuit is composed of twotemperature detection elements that are arranged in the vicinity of aheating element at upstream and downstream sides thereof, respectively,with respect to a forward flow of fluid for detecting the temperature ofsaid heating element, and a plurality of first resistors; a secondbridge circuit is composed of said heating element, a fluid temperaturedetection element arranged at a location free from the influence of heatfrom said heating element, and a plurality of second resistors; and aflow rate of said fluid is detected by using the fact that an amount ofheat transmitted from said heating element to said fluid in a statewhere said heating element is always held at a control temperaturehigher by a predetermined value than the temperature of said fluiddetected by said fluid temperature detection element depends on the flowrate of said fluid; said thermal flow sensor comprising: an amplifiersection that amplifies a voltage across opposite ends of at least one ofsaid plurality of second resistors; a current control section that iscontrolled based on an output voltage of said amplifier section; and anoutput terminal that is connected to one end of each of said twotemperature detection elements for outputting a detection resultcorresponding to the flow rate of said fluid; wherein said amplifiersection comprises an amplifier part that amplifies an input signal tosaid amplifier section, and an amplification factor control part thatcontrols an amplification factor of said amplifier part; saidamplification factor control part changes the amplification factor ofsaid amplifier part by means of an electric signal so that thetemperature of said heating element is adjusted to said controltemperature.
 11. A thermal flow sensor in which a bridge circuit iscomposed of two heating elements, two fluid temperature detectionelements, and a plurality of resistors; said two heating elements andsaid two fluid temperature detection elements are arranged in fluid, andsaid two fluid temperature detection elements are arranged at a locationfree from the influence of heat from said two heating elements; and aflow rate of said fluid is detected by using the fact that an amount ofheat transmitted from said two heating elements to said fluid in a statewhere said two heating elements are always held at a control temperaturehigher by a predetermined value than the temperature of said fluiddetected by said two fluid temperature detection elements depends on theflow rate of said fluid; said thermal flow sensor comprising: anamplifier section that amplifies a voltage across opposite ends of aseries circuit comprising at least one of said plurality of resistorsand one of said two fluid temperature detection elements; a currentcontrol section that is controlled based on an output voltage of saidamplifier section; and an output terminal that is connected to one endof said heating elements, which are controlled to be energized throughsaid current control section, for outputting a detection resultcorresponding to the flow rate of said fluid; wherein said amplifiersection comprises an amplifier part that amplifies an input signal tosaid amplifier section, and an amplification factor control part thatcontrols an amplification factor of said amplifier part; saidamplification factor control part changes the amplification factor ofsaid amplifier part by means of an electric signal so that thetemperatures of said heating elements are adjusted to said controltemperature.
 12. A thermal flow sensor in which a bridge circuit iscomposed of two heating elements, two fluid temperature detectionelements, and a plurality of resistors; said two heating elements andsaid two fluid temperature detection elements are arranged in fluid, andsaid two fluid temperature detection elements are arranged at a locationfree from the influence of heat from said two heating elements; and aflow rate of said fluid is detected by using the fact that an amount ofheat transmitted from said two heating elements to said fluid in a statewhere said two heating elements are always held at a control temperaturehigher by a predetermined value than the temperature of said fluiddetected by said two fluid temperature detection elements depends on theflow rate of said fluid; said thermal flow sensor comprising: anamplifier section that amplifies a voltage across opposite ends of oneof said two fluid temperature detection elements; a current controlsection that is controlled based on an output voltage of said amplifiersection; and an output terminal that is connected to one end of saidheating elements, which are controlled to be energized through saidcurrent control section, for outputting a detection result correspondingto the flow rate of said fluid; wherein said amplifier section comprisesan amplifier part that amplifies an input signal to said amplifiersection, and an amplification factor control part that controls anamplification factor of said amplifier part; said amplification factorcontrol part changes the amplification factor of said amplifier part bymeans of an electric signal so that the temperatures of said heatingelements are adjusted to said control temperature.
 13. A thermal flowsensor in which a bridge circuit is composed of at least one heatingelement, a fluid temperature detection element, and a plurality ofresistors; said heating element and said fluid temperature detectionelement are arranged in fluid, and said fluid temperature detectionelement is arranged at a location free from the influence of heat fromsaid heating element; and a flow rate of said fluid is detected by usingthe fact that an amount of heat transmitted from said heating element tosaid fluid in a state where said heating element is always held at acontrol temperature higher by a predetermined value than the temperatureof said fluid detected by said fluid temperature detection elementdepends on the flow rate of said fluid; said thermal flow sensorcomprising: a first amplifier section that is inserted in said bridgecircuit at a ground side thereof for amplifying a voltage acrossopposite ends of at least one of said plurality of resistors; a secondamplifier section that is inserted in said bridge circuit at a powersupply side thereof for amplifying a voltage across opposite ends of atleast other one of said plurality of resistors; a current controlsection that is controlled based on a voltage at one end of said heatingelement and a voltage at one end of said fluid temperature detectionelement; and an output terminal that is connected to one end of saidheating element, which is controlled to be energized through saidcurrent control section and said second amplifier section, foroutputting a detection result corresponding to the flow rate of saidfluid; wherein each of said first and second amplifier sectionscomprises an amplifier part that amplifies an input signal to said firstand second amplifier sections, and an amplification factor control partthat controls an amplification factor of said amplifier part; saidamplification factor control parts in said first and second amplifiersections change the amplification factors of said amplifier parts insaid first and second amplifier sections, respectively, by means of anelectric signal so that the temperature of said heating element isadjusted to said control temperature.